This proposal responds to the program announcement PAR-02-049 (R03) under the research objective #3. Alzheimer's Disease Drug Discovery. Title: Loss of synapses in physiological and pathological aging. Alzheimer's disease (AD) is a neurodegenerative disorder that is pathologically characterized by the presence of amyloid deposits composed by filaments of amyloid beta (Abeta) peptide. Abeta recombinant peptide (Abeta142) has the ability under different experimental conditions to assemble into fibrils or soluble forms such as protofibrils and small soluble oligomeric forms named ADDLs. Soluble Abeta oligomers have been shown to be neurotoxic as well as synaptotoxic and are believed to contribute to the neurodegeneration observed in AD. We propose that ADDLs are most likely involved in the loss of dendritic spines because 1) loss of synaptic terminals correlates well with the levels of soluble amyloid in humans and in an AD-transgenic mouse model without amyloid plaques, 2) ADDLs cause LTP dysfunction and neuronal death, 3) soluble amyloid has been detected in AD brain, 4) accumulation of oligomeric Abeta is age-dependent and concomitant with the onset of behavioral abnormalities in transgenic mice. We suggest that ADDLs impair synaptic plasticity by reducing the number of spines and by changing their shapes. In pilot data, we have demonstrated that ADDLs, rigorously prepared following our published protocol 102, induce synapse destabilization. Indeed, dissociated hippocampal cultures treated with 500nM ADDLs for 6 hours showed, using synaptophysin (SVP-38) immunolabeling, a decrease in SVP-38 positive puncta associated to an increase in synaptic bouton size compared to vehicle-treated cultures. PSD-95 immunoreactive spots also were enlarged. Aim 1 propose to study the effects of ADDLs on spine density and morphology and synapse integrity (matching of pre- and postsynapfic sites) in fluorescently-labeled organotypic hippocampal slice cultures. More than 90% of the excitatory synapses in the central nervous system occur on dendritic spines and changes in spine morphology have been implicated in learning and memory and in LTP. One important candidate in the spine morphology and plasticity is an activity-regulated cytoskeleton-associated protein named Arc whose mRNA is targeted to and translated at activated synapses. The disruption of Arc protein expression by antisense ODNs impairs the maintenance phase of LTP suggesting that Arc plays a fundamental role in the stabilization of activitydependent hippocampal plasticity. We propose in Aim2 to determine the involvement of Arc in the ADDLssensitive spine. This research will lay the groundwork for evaluating in novel in vitro screening and cell culture methods whether soluble Abeta forms are responsible for the synapse disorganization and dendritic spine loss observed in AD. Compounds found to inhibit Abeta oligomerization or the immuno-neutralization of soluble Abeta-derived toxins are anticipated to have therapeutic potential for the treatment of AD and related disorders 94. We have developed and identified a new class of antibodies directed against some oligomeric forms of Abeta that have the ability to neutralize the toxicity of soluble oligomers in cell culture 102. Therefore, the neutralization of ADDLs synaptotoxic effects by those oligomer-specific antibodies will be asses on the models described above.